Distributed temperature sensing systems (DTS) are

optoelectronic Optoelectronics (or optronics) is the study and application of electronic devices and systems that find, detect and control light, usually considered a sub-field of photonics. In this context, ''light'' often includes invisible forms of radiatio ...

devices which measure temperatures by means of

optical fibres An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means t ...

functioning as linear

sensors A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. A high accuracy of temperature determination is achieved over great distances. Typically the DTS systems can locate the temperature to a spatial resolution of 1 m with accuracy to within ±1°C at a resolution of 0.01°C. Measurement distances of greater than 30 km can be monitored and some specialised systems can provide even tighter spatial resolutions. Thermal changes along the optical fibre cause a local variation in the refractive index, which in turn leads to the inelastic scattering of the light propagating through it. Heat is held in the form of molecular or lattice vibrations in the material. Molecular vibrations at high frequencies (10 THz) are responsible for Raman scattering. Low frequency vibrations (10-30 GHz) cause Brillouin scattering. Energy is exchanged between the light travelling through the fibre and the material itself and cause a frequency shift in the incident light. This frequency shift can then be used to measure temperature changes along the fibre.

Measuring principle—Raman effect

Physical measurement dimensions, such as

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

and

tensile In physics, tension is described as the pulling force transmitted axially by the means of a string, a rope, chain, or similar object, or by each end of a rod, truss member, or similar three-dimensional object; tension might also be described as t ...

forces, can affect glass fibres and locally change the characteristics of light transmission in the fibre. As a result of the

damping Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples in ...

of the light in the quartz glass fibres through

scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...

, the location of an external physical effect can be determined so that the optical fibre can be employed as a linear sensor. Optical fibres are made from doped quartz glass. Quartz glass is a form of

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

(SiO₂) with amorphous solid structure. Thermal effects induce lattice oscillations within the solid. When light falls onto these thermally excited molecular oscillations, an interaction occurs between the light particles (

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

s) and the electrons of the molecule. Light scattering, also known as

Raman scattering Raman scattering or the Raman effect () is the inelastic scattering of photons by matter, meaning that there is both an exchange of energy and a change in the light's direction. Typically this effect involves vibrational energy being gained by a ...

, occurs in the optical fibre. Unlike incident light, this scattered light undergoes a spectral shift by an amount equivalent to the resonance frequency of the lattice oscillation. The light scattered back from the fibre optic therefore contains three different spectral shares: * the

Rayleigh scattering Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the ...

with the wavelength of the laser source used, * the Stokes line components from photons shifted to longer wavelength (lower frequency), and * the anti-Stokes line components with photons shifted to shorter wavelength (higher frequency) than the Rayleigh scattering. The intensity of the so-called anti-Stokes band is temperature-dependent, while the so-called Stokes band is practically independent of temperature. The local temperature of the optical fibre is derived from the ratio of the anti-Stokes and Stokes light intensities.

Measuring principle—OTDR and OFDR technology

There are two basic principles of measurement for distributed sensing technology, OTDR (Optical Time Domain Reflectometry) and OFDR (Optical Frequency Domain Reflectometry). For Distributed Temperature Sensing often a Code Correlation technology Method and apparatus for performing optical time domain reflectometry
, PatentOPTICAL REFLECTOMETRY ANALYSIS BASED ON FIRST ORDER AND SECOND ORDER SCATTER SIGNALS
Patent is employed which carries elements from both principles. OTDR was developed more than 20 years ago and has become the industry standard for telecom loss measurements which detects the—compared to Raman signal very dominant— Rayleigh backscattering signals. The principle for OTDR is quite simple and is very similar to the time of flight measurement used for

radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

. Essentially a narrow

laser pulse A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

generated either by semiconductor or solid state lasers is sent into the fibre and the backscattered light is analysed. From the time it takes the backscattered light to return to the detection unit it is possible to locate the location of the temperature event. Alternative DTS evaluation units deploy the method of Optical Frequency Domain Reflectometry ( OFDR). The OFDR system provides information on the local characteristic only when the backscatter signal detected during the entire measurement time is measured as a function of frequency in a complex fashion, and then subjected to

Fourier transformation A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed, ...

. The essential principles of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical backscatter signal. This is offset by the technically difficult measurement of the Raman scattered light and rather complex signal processing, due to the

FFT A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). Fourier analysis converts a signal from its original domain (often time or space) to a representation in the ...

calculation with higher linearity requirements for the electronic components. Code Correlation DTS sends on/off sequences of limited length into the fiber. The codes are chosen to have suitable properties, e.g.

Binary Golay code In mathematics and electronics engineering, a binary Golay code is a type of linear error-correcting code used in digital communications. The binary Golay code, along with the ternary Golay code, has a particularly deep and interesting connection ...

. In contrast to OTDR technology, the optical energy is spread over a code rather than packed into a single pulse. Thus a light source with lower peak power compared to OTDR technology can be used, e.g. long life compact semiconductor lasers. The detected backscatter needs to be transformed—similar to OFDR technology—back into a spatial profile, e.g. by

cross-correlation In signal processing, cross-correlation is a measure of similarity of two series as a function of the displacement of one relative to the other. This is also known as a ''sliding dot product'' or ''sliding inner-product''. It is commonly used fo ...

. In contrast to OFDR technology, the emission is finite (for example 128 bit) which avoids that weak scattered signals from far are superposed by strong scattered signals from short distance, improving the

Shot noise Shot noise or Poisson noise is a type of noise which can be modeled by a Poisson process. In electronics shot noise originates from the discrete nature of electric charge. Shot noise also occurs in photon counting in optical devices, where shot ...

and the signal-to-noise ratio. Using these techniques it is possible to analyse distances of greater than 30 km from one system and to measure temperature resolutions of less than 0.01°C.

Construction of Sensing Cable & System Integration

The temperature measuring system consists of a controller (

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

source, pulse generator for OTDR or code generator for Code Correlation or modulator and HF mixer for OFDR, optical module, receiver and micro-processor unit) and a quartz glass fibre as line-shaped temperature

sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

. The fibre optic cable (can be 70 km in length) is passive in nature and has no individual sensing points and therefore can be manufactured based on standard telecoms fibres. This offers excellent economies of scale. Because the system designer/integrator does not have to worry about the precise location of each sensing point the cost for designing and installing a sensing system based on distributed fibre optic sensors is greatly reduced from that of traditional sensors. Additionally, because the sensing cable has no moving parts and design lives of >30 years, the maintenance and operation costs are also considerably less than for conventional sensors. Additional benefits of fibre optic sensing technology are that it is immune to

electromagnetic interference Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electros ...

vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic function, periodic, such as the motion of a pendulum ...

and is safe for use in hazardous zones (the laser power falls below the levels that can cause ignition), thus making these sensors ideal for use in industrial sensing applications. With regards to the construction of the sensing cable, although it is based on standard

fibre optics An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means t ...

, care must be taken in the design of the individual sensing cable to ensure that adequate protection is provided for the fibre. This must take into account

operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...

(standard cables operate to 85 °C, but it is possible to measure up to 700 °C with the correct design), gaseous environment (

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

can cause deterioration of the measurement though " hydrogen darkening" aka attenuation of the silica glass compounds) and mechanical protection. Most of the available DTS systems have flexible system architectures and are relatively simple to integrate into industrial control systems such as

SCADA Supervisory control and data acquisition (SCADA) is a control system architecture comprising computers, networked data communications and graphical user interfaces for high-level supervision of machines and processes. It also covers sensors and ...

. In the oil and gas industry an

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...

based file standard (

WITSML {{Short description, Web development language WITSML is a standard for transmitting technical data between organisations in the petroleum industry. It continues to be developed by an Energistics facilitated Special Interest Group to develop XML stan ...

) has been developed for transfer of data from DTS instruments. The standard is maintained by

Energistics Energistics is a global, non-profit, industry consortium that facilitates an inclusive user community for the development, adoption and maintenance of collaborative, open standards for the energy industry in general and specifically for oil and ga ...

Laser Safety and Operation of System

When operating a system based on optical measurements such as optical DTS, laser safety requirements need to be considered for permanent installations. Many systems use low power laser design, e.g. with classification as laser safety class 1M, which can be applied by anyone (no approved laser safety officers required). Some systems are based on higher power lasers of a 3B rating, which although safe for use by approved laser safety officers, may not be suitable for permanent installations. The advantage of purely passive optical sensor technology is the lack of electric or electromagnetic interaction. Some DTS systems on the market use a special low power design and are inherently safe in explosive environments, e.g. certified to

ATEX directive The ATEX directives are two EU directives describing the minimum safety requirements for workplaces and equipment used in explosive atmospheres. The name is an initialization of the French term ''Appareils destinés à être utilisés en ATmos ...

Zone 0. For use in fire detection application, regulations usually require certified systems according to relevant standards, such as

EN 54 The EN 54 Fire detection and fire alarm systems is a series of European standards that includes product standards and application guidelines for fire detection and fire alarm systems as well as voice alarm systems. The product standards define ...

-5 or

-22 (Europe), UL521 or FM (USA), cUL521 (Canada) and/or other national or local standards.

Temperature estimations by using DTS

Temperature distributions can be used to develop models based on the Proper Orthogonal Decomposition Method or

principal component analysis Principal component analysis (PCA) is a popular technique for analyzing large datasets containing a high number of dimensions/features per observation, increasing the interpretability of data while preserving the maximum amount of information, and ...

. This allows to reconstruct the temperature distribution by measuring only in a few spatial locations

Applications

Distributed temperature sensing can be deployed successfully in multiple industrial segments: * Oil and gas production—permanent downhole monitoring, coil tubing optical enabled deployed intervention systems, slickline optical cable deployed intervention systems. * Power cable and transmission line monitoring ( ampacity optimisation) * Fire detection in tunnels, industrial conveyor belts and special hazard buildings * Industrial

induction furnace An induction furnace is an electrical furnace in which the heat is applied by induction heating of metal. Induction furnace capacities range from less than one kilogram to one hundred tons, and are used to melt iron and steel, copper, aluminum ...

surveillance * Integrity of

liquid natural gas Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane, C2H6) that has been cooled down to liquid form for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the vol ...

(LNG) carriers and terminals * Leakage detection at dikes and dams * Temperature monitoring in plant and process engineering, including transmission pipelines * Storage tanks and vessels More recently, DTS has been applied for environmental monitoring as well: * Stream temperature
Groundwater source detection and sediment scouring and deposition
* Temperature profiles in a mine shaft and over lakes and glaciers * Deep rainforest ambient temperature at various foliage densities * Temperature profiles in an underground mine, Australia * Temperature profiles in ground loop heat exchangers (used for ground coupled heating and cooling systems)

Power cable monitoring

ONLINE AMPACITY DETERMINATION OF A 220-KV CABLE USING AN OPTICAL FIBRE BASED MONITORING SYSTEM (Ji-Cable 2011)

Case study: Permanent Temperature Monitoring of a 220kV XLPE cable at the Olympic City 2008, Beijing

Case study: 33kV Distribution Cable monitored with DTS and modeled with RTTR (Real Time Thermal Rating)

Environmental monitoring

Tyler, S.W., J.S. Selker, M.B. Hausner, C.E. Hatch, T. Torgersen and S. Schladow. 2009. Environmental temperature sensing using Raman spectra DTS fiber optic methods. Water Resources Res
. * Selker, J.S., N. van de Giesen, M. Westhoff, W. Luxemburg, and M. Parlange. Fiber Optics Opens Window on Stream Dynamics. Geophysical Research Letters, , 2006 * Selker, J.S., L. Thévenaz, H. Huwald, A. Mallet, W. Luxemburg, N. van de Giesen, M. Stejskal, J. Zeman, and M. Westhoff, and M.B. Parlange.Distributed Fiber Optic Temperature Sensing for Hydrologic Systems. Water Resources Research, 42, W12202, , 2006
Tyler, S.W., S. Burak, J. McNamara, A. Lamontagne, J. Selker and J. Dozier. 2008. Spatially distributed temperatures at the base of two mountain snowpacks measured with fiber-optic sensors. Journal of Glaciology. 54(187):673-679

Pipeline leak detection

Gas Pipeline Leak and Ground Movement Detection Case Study

Sewer monitoring

* O.A.C. Hoes, R.P.S. Schilperoort, W.M.J. Luxemburg, F.H.L.R. Clemens and N.C. van de Giesen. Locating illicit connections in storm water sewers using fiber-optic distributed temperature sensing. Water Research, Volume 43, Issue 20, December 2009, Pages 5187-5197 * R.P.S. Schilperoort, F.H.L.R. Clemens, Fibre-optic distributed temperature sensing in combined sewer system, Water Science Technology. 2009;60(5):1127-34. doi: 10.2166/wst.2009.467. *Nienhuis J, de Haan CJ, Langeveld JG, Klootwijk M, Clemens FHLR. Assessment of detection limits of fiber-optic distributed temperature sensing for detection of illicit connections. Water Science and Technology. 2013;67(12):2712-8. doi: 10.2166/wst.2013.176 * Langeveld JG, de Haan CJ, Klootwijk M, Schilperoort RPS. Monitoring the performance of a storm water separating manifold with distributed temperature sensing.Water Science and Technology. 2012;66(1):145-50. doi: 10.2166/wst.2012.152. * Schilperoort RPS, Hoppe H, de Haan CJ, Langeveld JG. Searching for storm water inflows in foul sewers using fibre-optic distributed temperature sensing.Water Science and Technology. 2013;68(8):1723-30. doi: 10.2166/wst.2013.419. * movie which illustrates the application of DTS in sewers by use of the SewerOctopus of Royal HaskoningDH
Royal HaskoningDHV congratulates Egmond aan Zee with achieving the Blue Flag
(longer story) an
Riooloctopus, opsporen van foutieve aansluitingen vanuit het riool
(short movie of field work) *Mats Vosse, Rémy Schilperoort, Cornelis de Haan, Jaap Nienhuis, Marcel Tirion and Jeroen Langeveld, Processing of DTS monitoring results: automated detection of illicit connections, Water Practice and Technolog

Fire detection

Extensive fire tests prove use of DTS in road and rail tunnel (paper)

Paper published at International Conference on Automatic Fire Detection (AUBE'04); University of Duisburg

European Initiative SOLIT - Safety Of Life In Tunnels

Early detection of smoldering fire near conveyor belt systems (paper)

References

{{reflist Sensors Fiber optics Measuring instruments Fire detection and alarm Petroleum production Electrical components Power cables